1. Field of the Invention
The present invention generally relates to a Ball Grid Array (BGA) package and to a method of manufacturing the same. More particularly, the present invention relates to a BGA package which prevents moisture from penetrating to the semiconductor chip of the package and which effectively vents the heat generated by the chip.
2. Description of the Related Art
Semiconductor device packages are becoming faster, smaller, and thinner to meet the pressing demands for miniature and multi-functional electronic devices. A Ball Grid Array Package (hereinafter, referred to as a xe2x80x9cBGA packagexe2x80x9d) has been developed to meet these demands. The BGA package is a type of surface mount package which includes a printed circuit board (PCB), and solder balls (also referred to as solder bumps) instead of a lead frame for electrically connecting a semiconductor chip to the PCB. The BGA package has a high mounting density and thus employs a large number of I/O pins.
As described above, in the BGA package, a semiconductor chip is attached and electrically connected to the PCB. Circuit wiring patterns on a surface of the PCB are electrically connected to the semiconductor chip, which is mounted on the same surface of the PCB. The circuit wiring patterns are also electrically connected through signal via holes to external connections which are formed on the other surface of the PCB. Because the external connections are formed on the surface of the PCB which is opposed to that to which the semiconductor chip is mounted, the BGA package requires a mounting area that is smaller than that required by other conventional plastic packages. In conventional BGA packages, solder bumps serve as the external connections.
FIG. 1 is a cross-sectional view of a conventional BGA package and FIG. 2 is an enlarged view of thermal emissive via holes of a PCB of the package.
With reference to these figures, the conventional BGA package 100 has a semiconductor chip 20 electrically connected through a Printed Circuit Board (PCB) 10 to solder bumps 30. The solder bumps 30 serve as external connections for the BGA package 100.
The PCB 10 comprises a board body 19, and copper (Cu) pattern layers on an upper and a lower surface of the board body 19. The Cu pattern layers electrically connect the semiconductor chip 20 to the solder bumps 30. A plurality of signal via holes 14 penetrate the board body 19, in order to electrically connect the Cu pattern layer on the upper surface to the Cu pattern layer on the lower surface of the board body 19. Inner walls of the signal via holes 14 are plated with Cu.
The Cu pattern layer on the upper surface of the board body 19 comprises a chip attach area 60 and a plurality of circuit patterns 15. The chip attach area 60 refers to a region at which the semiconductor chip 20 is mounted to the PCB. The respective circuit patterns 15 are positioned around the chip attach area 60 and respective ends of the circuit patterns 15 are referred to as xe2x80x9cboard bonding pads xe2x80x9d. The board bonding pads 17 are electrically connected to the semiconductor chip 20 by bonding wires 40.
The Cu pattern layer on the lower surface of the board body 19 comprises a plurality of solder ball pads 13, to which solder bumps 30 are attached, and circuit patterns 15, which are electrically connected to the solder ball pads 13. Via holes, which are formed underneath the chip attach area 60, are referred to as xe2x80x9cthermal emissive via holes 62xe2x80x9d. The thermal emissive via holes 62 vent to the outside the heat generated during the operation of the semiconductor chip 20.
Both surfaces of the PCB 10 are coated with solder resist 16, except at locations corresponding to the board bonding pads 17 on the upper surface and the solder ball pads 13 on the lower surface. During the coating process, the thermal emissive via holes 62 are filled with the solder resist 16, as shown in FIG. 2.
After that, the upper surface of the PCB 10 is encapsulated with molding compound, such as a thermosetting resin, to protect the semiconductor chip 20 and the circuit patterns 15 from external environmental stress, whereby a package body 50 is formed. After attaching solder balls to the solder ball pads 13 of the lower surface of the PCB 10, the solder bumps 30 are formed by a reflow soldering process.
The conventional BGA package described above has the following problems. First, the PCB is extremely hygroscopic. Therefore, the BGA package absorbs moisture much more readily than do conventional packages using a metal lead frame.
Moisture is absorbed into the BGA package 100 through two routes: an absorption through the board body 19 and an absorption through the thermal emissive via holes 62. The amount of absorption through the board body 19 depends on the physical properties of the material of the board body 19. Therefore, problems caused by the absorption of the moisture through the board body 19 may be overcome by selecting a suitable material for the board body.
The thermal emissive via holes 62 provide the second route because the solder resist 16 with which they are filled is also extremely hygroscopic. Therefore, moisture can flow to the semiconductor chip 20 through the thermal emissive via holes 62. Such moisture affects the reliability of the package more seriously than the moisture absorbed through the board body 19.
The second problem is that although the thermal emissive via holes 62 are provided for transferring heat generated by the chip 20 to the outside of the package 100, the rate of heat transfer through the thermal emissive via holes 62 is considerably low.
More specifically, as shown in FIG. 2, the chip attach area 60 is relatively large and comprises a Cu layer 12. The thermal emissive via holes 62 are formed underneath this relatively large chip attach area 60, but the solder resist 16 filling the thermal emissive holes 62 has a low coefficient of thermal conductivity. Therefore, most of the heat is drawn by the Cu layer 12 at the chip attach area 60. The heat transfer rate is thus dictated in large part by the area of the Cu layer 12 at the chip attach area 60.
A third problem is that because the PCB is coated with solder resist 16, the viscosity of the solder resist 16 prevents the thermal emissive via holes 62 from being completely filled. In other words, the solder resist 16 may be left with voids 64. The voids 64 may cause the PCB to crack during reliability tests, such as a burn-in test which is carried out under high temperature and pressure after the BGA package 100 is manufactured.
Accordingly, an object of the present invention is to provide a BGA package which prevents moisture from penetrating to the chip through the thermal emissive via holes, and which possesses a comparatively high thermal emission property.
Another object of the present invention is to provide a BGA package having thermal emissive vias which are free of voids, and therefore is resistant to cracking.
The foregoing objects of the present invention are achieved by a BGA package having thermal emissive via holes filled with metal having a low melting point, excellent thermal conductivity and which is not highly hygroscopic, and by a method of manufacturing the same.
In a process of manufacturing a PCB of the BGA package of the present invention, the thermal emissive via holes are filled with metal having a low melting point while bottom ends of the thermal emissive via holes are closed by a coating of solder resist formed on the lower surface of a board body of the PCB. Metal paste, which comprises the metal having a low melting point, is forced through open top ends of the thermal emissive via holes using a screen-printing process, until the thermal emissive via holes are filled with the metal paste. A reflow soldering process causes the paste to completely fill the via holes.
Alternatively, instead of the metal paste, metal balls which have a low melting point, are positioned on the top ends of the thermal emissive via holes. The thermal emissive via holes are filled with the metal by carrying out a reflow soldering process. After that, solder resist is applied to the upper surface of the board body of the PCB.
After the PCB is produced, a semiconductor chip is attached to the PCB, chip pads of the semiconductor chip are connected to wiring patterns on one surface of the PCB with bonding wires, the bonding wires and the semiconductor chip are encapsulated to protect them from external environmental stress, and solder bumps are formed on wiring patterns on the other surface of the PCB.